The r-process of nucleosynthesis: overview of r-process sites Gail McLaughlin North Carolina State University Possible astrophysical sites of the r-process The r-process elements e. g. Uranium-238 Z=92, N=146 → need lots of neutrons A(Z,N)+ n ↔ A + 1(Z,N +1)+ γ Z − A(Z,N) → A(Z + 1,N − 1) + e +ν ¯e N rapid neutron capture as compared with beta decay Whats the most important criteria you are looking for? Whats the most important criteria you are looking for? What astrophysical sites have a lot of neutrons and eject material? How do you get neutrons? How do you get neutrons? 1. They already exist and just need to be liberated • in nuclei • in neutron stars 2. You make them through the weak interactions, i.e. conversion of protons into neutrons How do you judge a site? How do you judge a site? • plenty of neutrons • can populate halo stars • how often does it occur • does it match the abundance pattern Of course, there could be more than one site... Observational r-process data 1.00 Pt Zr 0.50 Sn Os Ga Ru Ba Cd Nd Dy Sr Gd 0.00 Er Pb Observational Halo Stars: Ce Sm Yb Ir −0.50 Ge Pd two r-process sites ε Y Hf Rh log −1.00 La Figure from Cowan and Sneden (2004) Mo Ag Ho Pr Nb Eu −1.50 Tb Au Tm main r-process and weak Lu Th −2.00 CS 22892−052 Abundances Upper Limits U r-process or multiple weak SS r−Process Abundances −2.50 30 40 50 60 70 80 90 Atomic Number Solar Abundances 0.1 scaled solar data 0.01 0.001 Abundance 0.0001 1e-05 40 60 80 100 120 140 160 180 200 A What would be your first guess? • Neutrino driven wind of the supernovae • Jets from core collapse supernovae • Accretion disks from core collapse supernovae • ONeMg supernovae • low entropy outflows from supernovae • He Shell of core collapse supernovae • Supernova with sterile neutrinos • Tidal ejection of neutron rich matter in neutron star mergers • shocked ejecta from merger • accretion disk outflows from mergers Possible astrophysical sites of the r-process • Neutrino Driven Wind of the Supernovae • Jets from Core Collapse Supernovae • Accretion Disks from Core Collapse Supernova • ONeMg Supernovae • low energy outflows from supernovae • Supernova with sterile neutrinos • Tidal ejection of neutron rich matter in neutron star mergers • shocked ejecta from merger • accretion disk outflows from mergers Compact object mergers figure from Surman 2008 figure from Korobkin 2012 Mergers have many signals • Gravitation wave signal, primary target of next generation detectors • Prime candidate for short duration gamma ray bursts • Huge emission of neutrinos, but hard to detect • optical signal powered by radioactive decay of newly formed elements • chemical evolution, elements produced in mergers, later observed in stars Interesting from a nucleosynthetic point of view, but also for many other reasons Evolution of neutron star merger • Insprial driven by gravitational wave emission • Until last moments of inspiral, neutron stars may essentially be treated as cold neutron stars • merger results in formation of a shocked extremely rapidly spinning hypermassive neutron star • later formation of a disk around a black hole • Models under development! Types of mass ejection • Dynamical ejection – material tidally ejected from tails – matter ejected through collisional region • Winds – accretion disk – hypermassive neutron star • Outflows from viscous heating What happens to all this ejecta from a nucleosynthesis perspective? Electron Fraction In order to get the r-process nuclei, prefer a lot of neutrons n Y = (1) e p + n Want this to be low. neutron stars start with low Ye. Of the types of outflow we have considered (dynamical, wind, viscous heating driven), which has lowest Ye? Dynamically ejected material from newtonian calculation 0.35 1.35–1.35M NS 1.35–1.35M NS 1.35–1.35M NS 0.3 o o o 0.25 0.2 0.15 0.1 Mass fraction 0.05 0 0.25 0.32 0.39 0.46 0.53 0.60 0.68 0.015 0.021 0.027 0.033 0.039 0.045 0.051 -0.4 0.2 0.8 1.4 2.0 2.6 0.35 1.20–1.50M NS 1.20–1.50M NS 1.20–1.50M NS 0.3 o o o 0.25 0.2 0.15 0.1 Mass fraction 0.05 0 0.25 0.34 0.44 0.53 0.63 0.72 0.82 0.015 0.025 0.035 0.045 0.055 0.065-1.0 -0.4 0.2 0.8 1.4 2.0 2.6 !/! Y log S S e Goriely et al 2011 Ye is so low you could have fission cycling! Why fission cycling is a good thing Basic observation Halo star data suggest that abundance pattern in 2nd & 3rd peak region is “robust”. Abundance pattern be- low 2nd peak shows variations between different stars. Need robust mechanism for populating 2nd & 3rd peaks. Fission Cycling? Note: Data show rare earth/3rd peak stable, few data in 2nd peak region. Generally assumed that 2nd/3rd also stable. Fission Cycling in the r-process 2.5 Symmetric Fission Asymmetric Fission 1 Asymmetric Fission + 2 n CS 22892−052 2 SS r−process Abundances 0 ν ↔ ν oscillations e s 1.5 130 −1 Y ÷ 195 1 Y −2 −3 0.5 Log Abundance −4 0 0 −1 −2 10 10 10 Y −5 e 30 40 50 60 70 80 90 100 Atomic Number abundance in 3rd/2nd peak as a fct of decreasing Ye Very little data on the relevant fission rates and daughter products Beun, GCM et al 2006, Beun, GCM et al 2008 Dynamically Ejected Material from Newtonian Calculation 100 -1 1.35-1.35M NS 10 Solar o 1.20-1.50M NS 10-2 o 10-3 10-4 10-5 Mass fraction 10-6 10-7 0 50 100 150 200 250 A Goriely et al 2011 Where is the evidence that there is fission cycling going on? What about calculations where neutrinos are included? 100 10-1 10-2 mass fraction 10-3 10-4 What has happened to the 0.0 0.1 0.2 0.3 0.4 0.5 Y Y e e? Why? 100 -1 10 Fig. from Wanajo et al 2014 10-2 mass fraction 10-3 10-4 0 10 20 30 S/k B How much stuff? Estimates depend on the hydrodynamics & thermodynamics & neutrino transport. Recent estimates: −3 • winds: ∼ 2 × 10 M⊙ Wanajo and Janka 2011 −2 −3 • tidal tail ejection: 10 to 10 M⊙ Goriely et al 2011, Korobkin et al 2012 −2 Need to make ∼ 10 M⊙ to account for all r-process material in Galaxy. Does it match the halo stars? Unresolved issues: 6−8 Mergers evolve slowly, τcoales ≈ 10 years. Not clear how to populate halo stars Mergers are rare, suggesting there should be more scatter in the amount of r-process material in halo stars than is seen. Does it match the halo stars? Argast et al 2004 Possible astrophysical sites of the r-process • Neutrino driven wind of the supernovae • Jets from core collapse supernovae • Accretion disks from core collapse supernovae • ONeMg supernovae • low entropy outflows from supernovae • He Shell of core collapse supernovae • Supernova with sterile neutrinos • Tidal ejection of neutron rich matter in neutron star mergers • shocked ejecta from merger • accretion disk outflows from mergers Core Collapse Supernovae • core unstable end of the life of a massive star Mcore ∼ 1.5Msun • Oxygen collapse to nuclear density C He Si H • core bounce • Fe core shock produced • shock stalls • neutrinos diffuse out of core, may energize shock Re-energizing the stalled shock Neutrinos heat the material below the stalled shock, helping along the two or three dimensional shock instabilities. From Blondin et al. R [km] Initial Phase of Collapse R [km] Neutrino Trapping (t ~ 0) R ~ 3000 R (t ~ 0.1s, c ~10¹² g/cm³) Fe Fe e !!!!!!!!! e !!!!!!!!!! !!!!!!!!! !!!!!!!!!! !!!!!!!!! !!!!!!!!!! !!!!!!!!! !!!!!!!!!! e !!!!!!!!! !!!!!!!!!! !!!!!!!!! ~ 100 !!!!!!!!!! !!!!!!!!! !!!!!!!!!! !!!!!!!!! !!!!!!!!!! !!!!!!!!! Si !!!!!!!!!! Si !!!!!!!!! !!!!!!!!!! !!!!!!!!! !!!!!!!!!! !!!!!!!!! e !!!!!!!!!! !!!!!!!!! Fe, !!!!!!!!!! Ni Fe, Ni !!!!!!!!! !!!!!!!!!! e !!!!!!!!! !!!!!!!!!! !!!!!!!!! !!!!!!!!!! !!!!!!!!! !!!!!!!!!! !!!!!!!!! !!!!!!!!!! !!!!!!!!! e !!!!!!!!!! !!!!!!!!! !!!!!!!!!! !!!!!!!!! !!!!!!!!!! !!!!!!!!! !!!!!!!!!! M(r) [M ] M(r) [M ] 0.5 1.0 ~ M Ch 0.5 Mhc 1.0 heavy nuclei Si−burning shell Si−burning shell R [km] Shock Propagation and Burst R [km] Bounce and Shock Formation e (t ~ 0.12s) R (t ~ 0.11s, c 2 o) R Fe Fe Rs ~ 100 km e radius of e shock R !!!!!!!!! formation !!!!!!!!! !!!!!!!!!! e !!!!!!!!! !!!!!!!!!! !!!!!!!!! !!!!!!!!!! !!!!!!!!! !!!!!!!!!! !!!!!!!!! position of !!!!!!!!!! !!!!!!!!! !!!!!!!!!! e !!!!!!!!! !!!!!!!!!! !!!!!!!!! shock !!!!!!!!!! !!!!!!!!! Si !!!!!!!!!! Si ~ 10 !!!!!!!!! e !!!!!!!!!! !!!!!!!!! !!!!!!!!!! !!!!!!!!! formation !!!!!!!!!! !!!!!!!!! !!!!!!!!!! !!!!!!!!! !!!!!!!!!! e !!!!!!!!! !!!!!!!!!! Fe !!!!!!!!! !!!!!!!!!! Fe, Ni !!!!!!!!! free n, !!!!!!!!!! !!!!!!!!! e !!!!!!!!!! !!!!!!!!! !!!!!!!!!! !!!!!!!!! p !!!!!!!!!! !!!!!!!!! !!!!!!!!!! Ni !!!!!!!!! !!!!!!!!!! e !!!!!!!!! !!!!!!!!!! !!!!!!!!! !!!!!!!!!! !!!!!!!!! !!!!!!!!!! !!!!!!!!! !!!!!!!!!! !!!!!!!!! !!!!!!!!!! !!!!!!!!! !!!!!!!!!! !!!!!!!!! !!!!!!!!!! M(r) [M ] 0.5 1.0 M(r) [M ] Nucleosynthesis0.5 1.0 in core collapse winds nuclear matter nuclear matter nuclei nuclei Si−burning shell Si−burning shell Shock Stagnation and Heating, R [km] Neutrino Cooling and Neutrino− R [km] Explosion (t ~ 0.2s) 10 5 Driven Wind (t ~ 10s) How much stuff? Rs ~ 200 4 ν ,ν −6 −4 e e 10 e,µ,τ e,µ,τ 10 -10 M⊙ Ni R ~ 100 3 g free n, p Si 10 Si Need (to account for all r-process p R ~ 50 2 He e 10 α n r−process? ν ,ν material): e e O e,µ,τ e,µ,τ e R ns ~ 10 R ν −6 M(r) [M ] n M(r) [M ] PNS1.3 gain layer 1.5 PNS 1.4 α, 9 3 M⊙ n, p ,n, Be, 10 cooling layer n, p α 12 C, seed Core collapse supernovae evolve “quickly” No problem with finding r-process elements in halo stars Core Collapse Supernovae: Nucleosynthesis in the Traditional Neutrino Driven Wind Hoped for r-process an site 0.1 scaled solar data -driven wind 0.01 ν 0.001 0.0001 1e-05 What happened? 1e-06 Entropy too high for outflow Abundance 1e-07 timescale (or vice versa) 1e-08 1e-09 1e-10 40 60 80 100 120 140 160 180 200 A Supernovae vs.